The present invention relates to a telephone system capable of judging kinds of various signals (line input signals) obtained through a telephone line.
A related art telephone system detects various signals (telephone line input signals) obtained through a telephone line by separately providing circuits for the respective signals. For example, in response to hanging up by a counterpart, an exchange equipment generates a calling party control (hereinafter called CPC) signal, a bell signal for calling, a usage state signal indicating a telephone line usage state, etc. The exchange equipment is provided with circuits individually for the respective signals.
A related art telephone system is described below with reference to the drawing. FIG. 2 is a circuit diagram showing a related art telephone system. The related art telephone system is constituted by a telephone line 21, a bell signal detecting circuit 22, a parallel telephone monitor circuit 23 for monitoring a telephone system connected in parallel, a CPC signal detecting circuit 24, a CPU 25 for processing a signal from each detecting circuit, a diode bridge 26 for rectification, a relay circuit 27 for capturing a telephone line, and a call-termination tone (hereinafter called CTT) detecting circuit 28.
The thus constructed related art telephone system operates as follows. A bell signal inputted through the telephone line 21 passes through a resistor and a capacitor of the bell signal detecting circuit 22, and turns on a transistor Tr1 of a photocoupler. In the case of responding to this bell signal, the CPU 25 closes the relay circuit 27 and instructs it to capture the telephone line.
When the telephone system captures the line and responds, if a telephone system connected in parallel is put into an off-hook state, a voltage (tip-ring voltage) XV between the tip and the ring of the telephone system changes. By this voltage change, the parallel telephone monitor circuit 23 detects the voltage change of the tip-ring voltage, and turns off a transistor Tr2. By this operation, an xe2x80x9cHxe2x80x9d level signal is inputted to an input terminal T2 of the CPU 25. On the other hand, when the parallel telephone system is put into an on-hook state, the tip-ring voltage increases, the transistor Tr2 is turned on, and an xe2x80x9cLxe2x80x9d level signal is inputted to the input terminal T2 of the CPU 25.
The CPU 25 detects inversion of logic of the signal inputted to the terminal T2, and judges that the parallel telephone system is put into an off-hook state. By this judgement, the telephone system cuts off the relay circuit 27 and shifts to a standby state.
Some exchange equipment cuts off the line for about 8 msec when a caller side makes an on-hook operation. A signal outputted at this time is a CPC signal (one of usage state signals), and a circuit for detecting this CPC signal is the CPC detecting circuit 24. In the CPC detection circuit 24, since a transistor Tr3 is normally turned on (during non-detection of the CPC signal), an xe2x80x9cLxe2x80x9d level signal is outputted to an input terminal T3 of the CPU 25. However, when the line is cut off, the transistor Tr3 is turned off, and an xe2x80x9cHxe2x80x9d level signal is outputted to the input terminal T3 of the CPU 25. When this xe2x80x9cHxe2x80x9d level signal is detected, the telephone system judges that the caller has hung up, and opens the telephone line. In the case of an automatic answering telephone set, when this xe2x80x9cHxe2x80x9d level signal is detected, the telephone system judges that the caller has hung up, so that recording is ended and the telephone line is opened.
Some exchange equipment does not output a CPC signal when a caller makes an on-hook operation but outputs a call-termination tone of from 40 to 600 Hz, or becomes quite silent. The call-termination tone detecting circuit 28 outputs an xe2x80x9cHxe2x80x9d (or xe2x80x9cLxe2x80x9d) level signal to an input terminal T4 of the CPU 25 when there is a sound, or outputs an xe2x80x9cLxe2x80x9d (or xe2x80x9cHxe2x80x9d) level signal to the input terminal T4 of the CPU 25 when there is no sound. The call-termination tone is either a continuous sound or a discontinuous sound having a constant period.
If a normal sound signal is inputted, the CTT detecting circuit 28 irregularly outputs an xe2x80x9cHxe2x80x9d level signal and an xe2x80x9cLxe2x80x9d level signal.
Thus, the CPU 25 monitors whether the output from the CTT detecting circuit 28 is
(a) fixed to an xe2x80x9cHxe2x80x9d level (at detection of continuous sound),
(b) fixed to an xe2x80x9cLxe2x80x9d level (at detection of no sound), or
(c) a signal periodically changing between the xe2x80x9cHxe2x80x9d level and the xe2x80x9cLxe2x80x9d level (at detection of discontinuous sound having a constant period).
In either one of the foregoing cases (a), (b), and (c), it is judged that a caller side has made an on-hook operation.
If the output from the CTT circuit 28 is a signal that irregularly changes between the xe2x80x9cHxe2x80x9d level and the xe2x80x9cLxe2x80x9d level, it is judged that the signal is a normal sound signal.
However, in the foregoing related art telephone system, since circuits are individually provided for different signals and detection is made, there has been a problem that the number of parts constituting the circuits is increased and a mounting area of a circuit substrate is enlarged. Besides, since all circuits are constituted by resistors, capacitors, transistors (photocouplers), and the like, there has been a problem that fluctuation in the case of mass production is large, and it is impossible to secure detection performance with high accuracy.
The present invention is aimed to provide a telephone system which solves the foregoing problems and enables reduction of the number of parts, detection with high accuracy, reduction of a mounting area, and reduction in costs.
In order to achieve this, a telephone system of the present invention comprises an A/D converter for making A/D conversion of a line input signal such as a CPC signal, a bell signal, and a usage state signal indicating a telephone line usage state to output a digital line input signal; and a central processing unit for judging a kind of the line input signal based on a line corresponding signal corresponding to the digital line input signal. By this construction, the telephone system of the invention enables reduction of the number of parts, detection with high accuracy, reduction of a mounting area, and reduction in costs.
The central processing unit of the telephone system of the invention judges the kind of the line input signal based on a voltage change value of the line corresponding signal or a frequency component thereof. By this judgement, in the telephone system of the invention, the kind of the line input signal can be accurately judged based on the line corresponding signal.